[WIP] Scheduler to replace chronograph

[benjaoming]

Summary

Replaces PR #4907

This is the initial framework for the scheduler, opening up the PR to let the rest of the team know what my thoughts were.

Terminology: Not sure if it should be named a scheduler, it might be more of a producer-consumer pattern, suggestions welcome.

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• Have tests been written for the new code? If you're fixing a bug, write a regression test (or have a really good reason for not writing one... and I mean really good!)
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file
• Add an entry to CHANGELOG.rst

Reviewer guidance

Comment on anything you like :)

Issues addressed

#4892

[benjaoming]

Could have been great :)